Asteroid 101: Classification of Asteroids, Their Position, Interactions with Earth, and More

In the previous article of this series, we learned how early astronomers first discovered asteroids. Along with the joy and disappointments associated with such discoveries, astronomers slowly realised that millions of such space rocks exist in our solar system. Nevertheless, they lived up to the challenge and identified as many as possible within a short period.

Amateur astronomers were also hunting for such minor bodies. As per the prevailing convention, if one finds a comet, their names are given to it. Similarly, if an individual finds an asteroid, then they were offered a chance to name it! That’s how we see not just Greek or Roman names but more inclusive naming to the newfound minor bodies.

When there are millions of objects in a belt, it gets difficult to look around in the vast data. So naturally, a scheme of classification had to be planned. Thankfully, astronomers did this quite early, and we started to differentiate asteroids based on their material properties or their orbital properties.

What are the types of asteroids?

The largest C-type Ceres is no more asteroid but now classified as Dwarf Planet. (NASA)

When I say material, it means the content of the asteroid. Scientists found that most of the asteroids are made up of similar material, and accordingly, three classes were created—C-type, S-type and M-type asteroids!

The C-type asteroids are the most abundant, with almost 70% of the total. These are the Chondrite type with clay and silicate abundance. These are the oldest asteroids in the solar system, and the material is similar to the original nebula—devoid of hydrogen and helium. If they stray and get on Earth, we get an irregular looking rock. Sometimes they are also carbonaceous.

C-type asteroids have very little albedo, which means that these asteroids are not very bright. Therefore, even to look at a large size C-type asteroid, individuals may need giant telescopes. The largest C-type is 10 Hygiea, with a diameter of around 430 km. Ceres, also C-type, is now promoted to a new class called dwarf planet!

Classic S-type asteroid Gaspra. The craters on the surface are impact craters. (NASA)

The S-type is the Stony type, consisting of nearly 17% of the total population and rich in silicates with embedded nickel-iron. These types of asteroids are relatively bright and can be seen using large binoculars or small telescopes. When found on Earth, they appear bright metallic objects and are one of the costliest meteorites on sale! The largest S-type asteroid 15 Eunomia is nearly 350 km in diameter.

The M-type are the so-called Metallic asteroids with moderate brightness. These are not so easy to classify and are the least studied type.

Position of asteroids in the cosmic world

Lagrangian Points L4 and L5 are stable. (Dr Abhay Deshpande)

Now, let us see the other form of classification, which depends on the position! The most prominent of course is the Main Asteroid Belt. Another essential group is called Trojan, along with other classes mentioned below.

Jupiter and Sun have a gravitational tug of war! Due to the basic nature of forces, there appear five points in the orbit where the gravitational forces appear to be balanced. These points are called Lagrangian points. In a nutshell, any object at these points will have nearly the same pull from Sun as well as Jupiter and hence will appear to be stable experiencing no effective pull.

Out of these five points, two are stable and can be used for purposes like placing a satellite. A classic example is India’s Aditya Mission which is expected to place a Solar Observation Satellite at one of the Lagrangian points of the Sun-Earth system. As the object is stable, it will need much less fuel.

Advertisement Similarly, any asteroid trapped in the Lagrangian point will be a stable object. At these points of L4 and L5 for the Jupiter-Sun system, around 250,000 asteroids are estimated to exist. At present, 7000 such asteroids are documented, with the largest one being 200 km in size.

Near-Earth Asteroids (NEA)

Distribution of asteroids and potentially hazardous asteroids (JPL/Caltech)

Being one of the most dangerous groups, Near-Earth Asteroids need careful observation. They are classified into four groups: Apollo, Atens, Atiras and Amor. The Apollo (named after the 1862 Apollo asteroid) has orbits larger than ours but still. The Atens (named after 2062 Atens) have orbits very close to the Earth’s orbit. Both these groups cross our path.

Amor (named after 1221 Amor) have orbits larger than Earth’s but often come very close to our home planet. On the other hand, Atiras (named after 163693 Atira) have much smaller orbits than that of the Earth’s and never cross our path.

Since these asteroids are potentially able to come near to our planet, there always remains a possibility that some of these could come sufficiently close to Earth. If the Earth’s gravity attracts any of these asteroids, they may change the path and trigger a possible impact on the planet. Hence, all these 20,000 plus objects are constantly monitored to ensure they are not on a collision path to Earth.

What happens when the asteroid comes towards Earth?

Meteors fall during dark nights. (Navicore)

If any rock from outer space comes zooming towards Earth, it comes in contact with the planet’s atmosphere and burns out as it travels towards us. We call it a meteor, but many people refer to it incorrectly as a falling star! It creates a bright streak of light in the sky and leaves behind a brilliant trail.

The brilliance of flash depends on the size and composition of the incoming rock. If a large enough asteroid hits us, it can create substantial devastation. In the following article, we will read about the hazards and issues related to such possible impacts.

This article is a part of a series of three articles on Asteroid Day, published each day from June 30. Here is the last article of this series:

A Brief History of How Asteroids Were First Identified and Classified by Astronomers of 19th Century

Dr Abhay Deshpande is a Senior Scientist (Physicist) working for SAMEER, R&D Lab of MeitY, Government of India. He is also the Honorary Secretary of Khagol Mandal, a non-profit collective of astronomy enthusiasts who organise various sky observation programmes, lectures and study tours.

This article is a guest column reflecting the author’s opinions and does not necessarily represent the official views of The Weather Channel.

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